Prepolymers of olefins containing a chromiumoxide and a granular refractory oxide support

ABSTRACT

The present invention relates to a process for the polymerization of ethylene or copolymerization of ethylene and at least one other alpha-olefin, in the gas phase in a fluidized bed reactor, in the presence of a catalyst comprising chromium compounds associated with a granular support comprising a refractory oxide and activated by thermal treatment, which process is characterized in that the catalyst is used in the form of active particles of prepolymer, obtained by bringing the said catalyst in contact with ethylene, alone, or in a mixture with at least one higher alpha-olefin, in such a way that the prepolymer contains from 4×10 -5  to 3 milligram atoms of chromium in the catalyst per gram.

This application is a division of application Ser. No. 06/776,470 filedSep. 16, 1985, now U.S. Pat. No. 4,910,271.

The present invention relates to a process for the polymerisation ofethylene or copolymerisation of ethylene or alpha-olefin in the gasphase by means of a catalyst comprising a chromium oxide associated witha granular support comprising a refractory oxide and activated by heattreatment, for example a "Phillips" type catalyst.

It is already known that ethylene can be polymerised alone or in amixture with alpha-olefins in the presence of catalyst comprising achromium oxide compound associated with a granular support comprising arefractory oxide and activated by heat treatment. These catalysts, usedin the polymerisation of alpha-olefins and of ethylene in particular,have been described in numerous patents, such as for example BritishPatent Nos. 790195 and 804641.

It is also known that such catalysts must preferably comprise at leastone chromium compound with a valency equal to 6, in a quantity such thatthe said catalyst contains at least 0.05% by weight of chromium.However, it has been discovered that the polymerisation of thealpha-olefins may also be carried out under good conditions by means ofchromium compounds with varying valencies, mostly less than 6, thesecompounds being obtained for example by reducing hexavalent chromiumcompounds before the commencement of polymerisation and/or during thepolymerisation itself.

It is known that these catalysts may be prepared by depositing achromium compound such as a chromium oxide, or a chromium compound whichcan be converted by calcing into chromium oxide, on a granular supportcomprising a refractory oxide, then by activation by thermal treatmentat a temperature of at least 250° C. and at most equal to thetemperature at which the granular support begins to sinter, so that atthe end of the thermal treatment the chromium compound is at leastpartially in the hexavalent state. There also exist numerous methods formodifying these catalysts, especially by incorporating in them titanium,fluorine or organometallic compounds.

It is also known that alpha-olefins can be polymerised in the gas phasein the presence of such catalysts, especially under a pressure less than4 MPa and in a fluidised bed reactor in which the solid polymer beingformed is maintained in the fluidised state by means of a rising currentcomprising of a gas mixture consisting basically of ethylene andpossibly of alpha-olefins to be polymerised. The gas mixture leaving thereactor is generally cooled before being recycled into the reactor, andan additional quantity of ethylene and possibly alpha-olefins is addedto it, corresponding to the quantity consumed. The velocity offluidisation in the fluidised bed reactor must be sufficiently high toguarantee the homogenisation of the fluidised bed, and to eliminateeffectively the heat produced by the polymerisation reaction. Thecatalyst may be introduced continuously or semicontinuously into thefluidised bed reactor. Withdrawal of the polymer produced may also beperformed continuously or semicontinuously. Thus, various methods ofproduction in the gas phase of polyolefins in the presence of chromiumbased catalysts have already been described in numerous patents, such asfor example British patents Nos. 810948, 1014205 and 1391771 , and U.S.Pat. Nos. 2,936,303, 3,002,963, 3,023,203 and 3,300,457.

However, experience has shown that a certain number of difficultiesappear in controlling the polymerisation reaction in the gas phase whencatalysts of this type are employed. More especially, in a fluidised bedpolymerisation process, it is important to control the heat exchange ofthe polymerisation reaction, particularly by controlling the velocity ofthe rising current providing the fluidisation, and for this reason theform and dimensions of the catalyst particles are important parameters.Thus, when it is desired to increase the fluidisation velocity tocomparatively high values, such as for example comprised between 40 and120 cm/sec, with a view to increasing the yield of the polymerisationreaction, the particles of a catalyst of this type which are introducedinto the fluidised bed reactor are too fine and are inevitably entrainedoutside the fluidised bed, thus causing undesirable reactions outsidethis bed.

It has already been proposed to use supported catalysts based onchromium oxide, the particles of which are sufficiently large to avoidthe above mentioned drawback. However, having in view the fact thatpolyolefin granules must have a relatively limited size, in order toavoid difficulties during the polymerisation operation, particularly ina fluidised bed reactor, the chromium content in the suggested catalystsmust be low, so that the chromium content in the polyolefins couldremain below a certain limit beyond which colour and/or odour problemscould arise. When the chromium content in the supported catalysts islow, gel problems resulting from the relatively high content of supportin the polyolefins could however be encountered. Moreover, catalysts ofthis type can easily be poisoned by the impurities which are present insmall quantities in the gaseous mixture.

It has also been proposed to polymerise olefins in the gas phase, bymeans of supported catalysts based on a metal oxide such as a chromiumoxide, by contacting the catalyst with an aluminium hydride or analkylaluminium in an inert solvent and introducing directly the mixtureunder pressure into the polymerisation reactor.

Such a method, which has been discussed in the French patent No.2059217, cannot be efficiently operated at the industrial scale withsupported catalysts based on chromium oxide, by reason of the behaviourof this type of catalyst after it has been brought into contact witholefins.

Experience has also shown that whatever the process of polymerisationused, the chromium based catalysts have an initial activity of nil orclose to nil, with an induction period which renders difficult not onlythe start-up of the polymerisation reaction, but also the maintenance ofconstant polymerisation conditions in the reaction medium, such as thetemperature. When the polymerisation is started, the catalyst activityrapidly accelerates, which renders the polymerisation reaction in a gasphase, particularly in a fluidised bed, difficult to control. Therefore,if the previous contact between the catalyst and an aluminium hydride oran alkylaluminium is long enough to activate the catalyst, the lattercould provoke rapid and random variations in the polymerisationconditions in the reaction medium which involve considerable risks ofhot spots and bursting into fine particles of the grains forming thefluidised bed. In particular, these hot spots may cause the formation ofagglomerates, and setting of the polymer in the interior of thefluidised bed, generally involving the stoppage of polymerisation.

The present invention relates to an improved process for thepolymerisation of ethylene or copolymerisation of ethylene and one ormore alpha-olefins in the gas phase by means of a fluidised bed, using acatalyst comprising a chromium oxide associated with a granularrefractory oxide support and activated by thermal treatment, thecatalyst being in the form of active particles of prepolymer havingsuitable particle size characteristics especially suited to therelatively high speed of fluidisation and possessing an initialcatalystic activity such that favourable conditions of polymerisation orcopolymerisation are achieved.

The present invention therefore relates to a process for thepolymerisation of ethylene, or the copolymerisation of ethylene and atleast one higher alpha-olefin comprising 3 to 12 carbon atoms, in thegas phase in a fluidised bed reactor, in the presence of a catalystcomprising a chromium compound associated with a granular refractoryoxide support which catalyst has been activated by thermal treatment ata temperature of at least 250° C. and at most equal to the temperatureat which the granular support begins to sinter, the activation beingcarried out in a non-reducing atmosphere, preferably an oxidisingatmosphere, which process is characterised in that the catalyst is usedin the polymerisation or copolymerisation in the form of catalyticallyactive particles of prepolymer which are obtained during a separateprepolymerisation operation comprising bringing the said catalyst intocontact with ethylene, or with a mixture of ethylene and at least onehigher alpha-olefin comprising from 3 to 12 carbon atoms, during aperiod of time such that the produced prepolymer contains from 4×10⁻⁵ to3 and preferably from 10⁻³ to 10⁻¹ milligram atoms of chromium per gram,and separating the prepolymer from the unreacted quantity of ethylene orthe mixture of ethylene with at least one higher alpha olefin.

It has surprisingly been found that the prepolymers prepared and usedaccording to the invention, which have been separated from the unreactedquantity of ethylene or of the mixture of ethylene with at least onehigher alpha olefin, possess an initial catalytical activitysubstantially less than the same prepolymer which has not been separatedfrom the unreacted olefins. During the course of polymerisation orcopolymerisation, the catalytical activity of the prepolymersprogressively increases to the activity level of the unseparatedprepolymer. This permits the avoidance of the above mentioned drawbacks,which could lead to the formation of agglomerates or fine particles ofpolymers or copolymers, and facilitates the production of polymers orcopolymers having the desired properties and with a relatively lowcontent of catalyst residues.

The catalysts used according to the invention can be obtained by a largenumber of known processes, especially by those according to which, in afirst stage, a chromium compound such as a chromium oxide generally ofthe formula CrO₃, or a chromium compound which can be converted bycalcining into chromium oxide, such as for example a chromium nitrate orchromium sulphate, an ammonium chromate, a chromium carbonate, achromium acetate, a chromium acetylacetonate or a tertbutyl chromate, isassociated with a granular support based on a refractory oxide such as,for example, silica, alumina, zirconium oxide, thorium oxide, titaniumoxide or mixtures or coprecipitates of two or several of these oxides.In a second stage, the chromium compound associated with the granularsupport is activated by thermal treatment at a temperature of at least250° C. and at most equal to a temperature which the granular supportbegins to sinter; the temperature of the thermal treatment is generallycomprised between 250° and 1200° C., and preferably comprised between350° and 1000° C. This thermal treatment is carried out under anon-reducing atmosphere, preferably an oxidising atmosphere, generallyconsisting of a gas mixture comprising oxygen, such as for example air.The duration of the thermal treatment may be between 5 minutes and 24hours, preferably between 30 minutes and 15 hours so that at the end ofthis treatment the chromium compound is at least partially in thehexavalent state. The content by weight of chromium of the catalyst thusobtained is generally comprised between 0.05 and 30%, and preferablybetween 0.1 and 3%.

The granular supports based on a refractory oxide used in thepreparation of catalysts according to the invention generally occur inthe form of solid particles which preferably have a mean diameter bymass comprised between 20 and 300 microns. These granular supports maybe obtained by various known processes, particularly by theprecipitation of silicon compounds, such as silica, from a silicatesolution of an alkali metal, or else by coprecipitation of a gel orhydrogel of refractory oxide from solutions comprising at least twocompounds chosen from amongst compounds of silicon titanium, zirconium,thorium or aluminium. Such processes are described in particular in U.S.Pat. Nos. 4,053,436 and 4,101,722. Another method for preparing thecatalysts according to the invention consists in preparing a support ofsilica and of titanium oxide by calcining a compound of titanium, suchas tetraisopropoxide of titanium, deposited on a silica, at atemperature comprised between 500° and 900° C. in an atmosphere of dryair, impregnating the support with a chromium compound, such as tertiarybutyl chromate and activating the resulting product by heat treatment.Such a method is discussed, for example in the U.S. Pat. No. 3,879,362.

One may also use a catalyst obtained in a first stage by coprecipitatingat least one refractory oxide, such as silica or alumina and a chromiumcompound to form a co-gel. In a second stage this co-gel is dried andthereafter activated by thermal treatment.

Another technique for preparing catalysts which can be used according tothe invention consists in carrying out the activation in the presence ofcertain titanium compounds, such as titanium tetraisopropoxide,according to a method described for example in French patent 2134743.

Another preferred method for preparing the catalysts according to theinvention consists in performing the activation in the presence offluorine compounds chosen among hexafluorotitanate, tetrafluoroborate orhexafluorosilicate of ammonium, possibly in the presence of a titaniumcompound chosen among titanium alcoholates, this method enablingadvantageously to increase the activity of these catalysts or to modifythe properties of the polyolefins produced. For example, the U.S. Pat.No. 3,130,183 describes a supported catalyst based on chromium oxide,this catalyst being activated in the presence of fluorine compound, suchas an ammonium fluorosilicate. Another method described in Britishpatent 1391771 consists in performing the activation of the supportedcatalyst based on chromium oxide in the presence on the one hand of atitanium compound, such as a titanium alcoholate, for example titaniumtetraisopropoxide and on the other had of a fluorine compound, such asammonium hexafluorotitanate, ammonium tetrafluoroborate or ammoniumhexafluorosilicate. The content by weight of fluorine in these catalystsis preferably comprised between 0.05 and 8% by weight.

The catalysts may also be modified by bringing them into contact withorgano-metallic compounds, such as an alkylaluminium alcoholate asdescribed for example in French patent 2269537.

According to the invention it is preferred to use catalysts comprising achromium compound and a titanium compound to produce polymers orcopolymers of ethylene having in particular a comparatively low meanmolecular weight. The content by weight of titanium of these catalystsis generally comprised between 0.1 and 20% and preferably between 0.5and 8%.

According to the invention the prepolymers are obtained bypolymerisation of ethylene, or a mixture of ethylene with at least onehigher alpha-olefin comprising 3 to 12 carbon atoms in the presence ofthe defined catalyst. As higher alpha-olefins to be used in a mixturewith ethylene it is preferred to use propylene, 1-butene, 1-hexene,4-methyl-1-pentene or 1-octene. The prepolymerisation may be effectedeither in suspension in a liquid hydrocarbon medium or in the gas phasein a fluidised bed reactor, or in a reactor provided with a mechanicalstirrer, or in a fluidised bed reactor provided with a mechanicalstirrer, at a temperature which is lower than the temperature at whichthe prepolymer particles begin to soften and to form agglomerates,preferably at a temperature between 40° and 115° C.

The prepolymerisation is stopped when the prepolymer contains from4×10⁻⁵ to 3, and preferably from 1×10⁻³ to 10⁻¹ milligram atoms ofchromium per gram, the particles of the prepolymer having a meandiameter by mass comprised, preferably between 40 and 1000 microns, morepreferably between 80 and 500 microns. The above mentioned chromiumcontent and dimensions are very suitable for polymerisation is afluidised bed.

The prepolymerisation may be carried out in two or more stages ifdesired. In this case the first prepolymerisation stage (which isreferred to as the stage of coating the catalyst) is carried out by thepolymerisation of ethylene or copolymerisation of ethylene and at leastone higher alpha-olefin in a liquid hydrocarbon medium. Generally thisstage may be continued until the coated catalyst obtained contains from0.1 to 10 g of polymer or copolymer per milligram atom of chromium. Thesecond prepolymerisation stage can be performed either in a liquidhydrocarbon medium or in the gas phase; generally this stage iscontinued, until the prepolymer contains from 4×10⁻⁵ to 3, andpreferably from 1×10⁻³ to 10⁻¹ milligram atoms of chromium per gram. Thedimensions of the particles of the prepolymer are preferably comprisedin the limits which have been recited above.

The prepolymerisation may be carried out advantageously in the presenceof at least one organometallic compound (a) of a metal of groups I toIII of the Periodic Table of Elements. The organometallic compound (a)is chosen for preference from amongst the organo-aluminium,organo-magnesium and organo-zinc compounds or a mixture of thesecompounds. As organo-aluminium compounds, one may select atrialkylaluminium, a hydride or an alcoholate of alkylaluminium.Preferably a trialkylaluminium is used such as triethylaluminium, or analcoholate of alkylaluminium such as diethylaluminium ethoxide. Thequantity of organo-metallic compound (a) used in prepolymerisation issuch that the atomic ratio of the quantity of metal in the saidorgano-metallic compound (a) to the quantity of chromium in the catalystis comprised between 0.01 and 30 and preferably between 0.1 and 1.

The organo-metallic compound (a) is used advantageously during theprepolymerisation, in order to improve the start-up of theprepolymerisation, in order to improve the start-up of theprepolymerisation reaction and especially to reduce, or to eliminate,the induction period during this reaction. Moreover, experience hasshown that in certain cases the presence of an organometallic compound(a) such as an alkylaluminium alcoholate during the prepolymerisationmakes it possible to reduce substantially the formation of waxes whichare mainly constituted of polymers or copolymers soluble in n-hexane at60° C. and which tend to render the particles of prepolymer sticky andto alter their polymerisation activity. Therefore, it is particularlyadvantageous to prepare prepolymers having a content of polymer orcopolymer soluble in n-hexane at 60° C. of less than or equal to 2.0% byweight.

The prepolymerisation may also be performed in the presence of hydrogento control the molecular weight of the prepolymer produced, especiallyif the latter is obtained in the presence of the organometallic compound(a).

The prepolymer is separated from the unreacted quantity of ethylene orfrom the mixture of ethylene with at least one higher olefin. Thisoperation can be performed either by degassing the polymerisation mediumin which the prepolymer is prepared, care being taken that no impuritieswhich would impair the catalyst, such as air or moisture enter intocontact with the prepolymer, or by stopping the introduction of ethyleneor of the mixture of ethylene with at least one higher olefin, until nosubstantial quantity of ethylene or of the mixture remains in thepolymerisation medium.

Before using the prepolymer in the polymerisation of ethylene or thecopolymerisation of ethylene with at least one higher olefin, it isadvantageous to perform one or more of the following subsidiaryoperations.

It is favourable to submit the prepolymer, before it is used in thefluidised bed polymerisation reactor, to one or more extractionoperations by means of a liquid hydrocarbon such as n-hexane. Theseextraction operations make it possible to eliminate the waxes which mayform during prepolymerisation and which consist essentially of polymersor copolymers soluble in n-hexane at 60° C. These waxes are particularlyundesirable during the drying of the prepolymers, because they tend tostick the particles of prepolymer together and to form agglomerates. Inthis way these waxes may be eliminated efficiently from the prepolymerparticles which may have a content or polymer or copolymer soluble inn-hexane at 60° C. of less than or equal to 2.0% by weight. Theseextraction operations also make it possible to create porosity insidethe prepolymer particles, and in particular they favour theaccessibility of the ethylene and the higher alpha-olefins to thecatalyst sites. This porosity also makes it possible to impregnate theprepolymer more efficiently with liquid compounds, such asorganometallic compounds, and thus to protect the active sites betteragainst possible poisons.

It is also possible to add an organometallic compound (b) to theprepolymer, at the end of the preparation of the prepolymer, asexplained further into detail in this disclosure.

Finally, the prepolymer is preferably obtained as a dry powder, whichmeans that it is preferably separated from the liquid hydrocarbon mediumin which it may have been prepared.

The prepolymers, which contain the active chromium based catalyst, arebrought into contact with ethylene or a mixture or ethylene and at leastone higher alpha olefin under polymerisation or copolymerisationconditions in the gas phase in a fluidised bed reactor, preferably inthe presence of an organometallic compound (b) of a metal of Groups I toIII of the Periodic Table of Elements. As organometallic compound (b),one may use organoaluminium, organomagnesium and organozinc compounds ora mixture of these compounds, and especially trialkyl aluminiums,hydrides or alcoholates of alkyl aluminium. The organometallic compound(b) may be identical to or different from organometallic compound (a).

The presence of the organometallic compound (b) during thepolymerisation or copolymerisation in the gas phase in a fluidised bedreactor makes it possible to increase advantageously the yield of thereaction. In particular it makes it possible to destroy the poisonspresent in the reaction medium and also to improve the control of themean molecular weight, and the distribution of the molecular weights ofthe polymer or copolymer produced.

The organometallic compound (b) may be added to the prepolymer at theend of prepolymerisation, preferably after the extraction operations bymeans of a liquid hydrocarbon, but before the introduction of the saidprepolymer into the fluidised bed reactor. In particular it may be addedto the prepolymer which has been previously placed in suspension in theliquid hydrocarbon, such as n-hexane, and in this case introduced in thepure state into the said suspension. The prepolymer is finally obtainedin the form of a powder after evaporating the liquid hydrocarbon.

It is also possible to introduce the organometallic compound (b) directinto the fluidised bed reactor, independently of the prepolymer. In thiscase it is particularly advantageous to dissolve the organometalliccompound (b) beforehand in a saturated aliphatic hydrocarbon comprising,for example, from 4 to 7 carbon atoms, in order to facilitate itsdispersion in the fluidised bed, and its contact with the prepolymerintroduced elsewhere.

The organometallic compound (b) may also be used advantageously bycombining the two previous methods. In particular it may be added inpart to the prepolymer prior to its introduction into the fluidised bedreactor and in part introduced separately from the prepolymer into thefluidised bed reactor. These two parts of organometallic compound (b)may be of an identical or different nature.

Whatever the method employed for using the organometallic compound (b),the latter is used in a quantity such that the atomic ratio of thequantity of the said organometallic compound (b) to the quantity ofchromium in the catalyst contained in the prepolymer is at most equal to100, preferably comprised between 0.1 and 20 and more especiallycomprised between 0.5 and 4. As a consequence, the total quantity oforganometallic compounds (a) and (b) is such that the atomic ratio ofthe quantity of metal in the said organometallic compounds to thequantity of chromium in the catalyst is at most equal to 130 andpreferably comprised between 0.1 and 21.

It has also been found, which constitutes a part of the invention, thatthe presence of the organometallic compound (b) during thepolymerisation or copolymerisation in the gas phase makes it possible tocontrol the mean molecular weight of the polymer or the copolymer, inthe absence or in the presence of hydrogen in the gaseous mixturecontaining ethylene or a mixture of ethylene and at least one higheralpha olefin. In the absence of hydrogen, the mean molecular weight ofthe polymer or the copolymer is increased when the atomic ratio of theorganometallic compound (b) to the quantity of chromium in the catalystcontained in the prepolymer is increased. This corresponds to a decreaseof the melt flow ratio of the polymer or the copolymer when the aboveratio is increased. Surprisingly, it has been found that in the presenceof hydrogen, and of the organometallic compound (b), the mean molecularweight of the polymer or the copolymer is decreased when the ratio ofhydrogen to ethylene in the gaseous mixture is increased, which meansthat the melt flow ratio of the polymer or the copolymer increases whenthe proportion of hydrogen in the gaseous mixture is increased, allother parameters remaining the same. This result is unexpected, since itis known that hydrogen does not show any substantial effect on the meanmolecular weight of polymer obtained in the presence of supportedchromium based catalysts. The useful values of the above mentionedratios are defined further in this disclosure.

Bringing the prepolymer into contact with ethylene or with a mixture ofethylene and at least one higher alpha olefin in the conditions ofpolymerisation or copolymerisation in a fluidised bed is performed bytechniques which are in themselves known. In particular, the gaseousmixture comprising ethylene and possibly higher alpha-olefin to bepolymerised flows upwardly through the fluidised bed, which consists ofparticles of polymer or copolymer in course of formation. The ethyleneand possibly the higher alpha-olefin are introduced into the fluidisedbed reactor at a temperature such that the reaction medium is at atemperature of at least 50° C. and advantageously at least 80° C., butlower than the temperature at which the particles of polymer or ofcopolymer begin to soften and to from agglomerates, preferably lowerthan 115° C.

The velocity of fluidisation of the gaseous mixture is preferablycomparatively high, so as to guarantee good homogenisation of thefluidised bed, without having recourse to any other means ofhomogenisation, especially mechanical, in order to eliminate efficientlythe heat released by polymerisation and to increase the yield ofpolymerisation. This fluidisation velocity is generally comprisedbetween 3 and 10 times the minimum velocity which is necessary forfluidising the particles of polymer or copolymer in the course ofgrowth, in most cases the gas velocity is comprised between about 40 and120 cm/sec, preferably comprised between 50 and 100 cm/sec and moreespecially comprised between 60 and 90 cm/sec. In flowing through thefluidised bed only a part of the ethylene and any higher alpha-olefinpolymerises in contact with the particles of polymer or copolymer in thecourse of growth. The fraction of ethylene and any alpha-olefin which isnot reacted leaves the fluidised bed and passes through a coolingsystem, in order to eliminate the heat produced during the reaction,before being recycled into the fluidised bed reactor, by means of acompressor.

The total pressure in the reactor may be in the vicinity of atmosphericpressure, but is preferably higher in order to increase thepolymerisation speed. It may be comprised between 0.5 and 4 MPa, andpreferably comprised between 0.8 and 3 MPa.

In order to control, and in particular to reduce the molecular weight ofthe polymers or copolymers produced, it is possible to vary the ratio oforganometallic compound (b) to the quantity of chromium in the catalystcontained in the prepolymer, in the above mentioned range and possiblyto add to the gas mixture circulating in the fluidised bed reactorhydrogen in a quantity such that the molar ratio of hydrogen to ethyleneand any higher alpha-olefin is not greater than 5 and is preferablycomprised between 0.1 and 1. For the same purpose, it is also possibleto vary the temperature of polymerisation.

The gas mixture may also comprise an inert gas in a quantity such thatthe molar ratio of inert gas to ethylene and any higher alpha-olefin isnot greater than and is preferably comprised between 0.1 and 2. Thisinert gas may be chosen from amongst nitrogen, methane, ethane, propane,butane, isobutane or a mixture of these gases. In particular it makes itpossible to improve appreciably the elimination of reaction heat and tomodify favourably the kinetics of the polymerisation.

Furthermore the gas mixture may comprise one or more higher alpha-olefincomprising from 3 to 12 carbon atoms, preferably chosen from amongstpropylene, 1-butene, 1-hexane, 4-methyl-1-pentene and 1-octene.

The polymer can be removed from the reaction vessel by means of variousmechanical devices. The preferred device comprises providing the lowerportion of the reaction vessel with an aperture which is capable ofbeing closed and which communicates with a chamber having a pressurelower than that in the reaction vessel. Opening the aperture for a giventime makes it possible to remove into the chamber the desired amount ofpolymer. When the apparatus is closed, the chamber can be brought intocommunication with the exterior for the polymer to be collected.

The process of the invention is preferably carried out in a manner suchthat the operating conditions of the reaction vessel are substantiallyconstant. This mode of operation can be achieved, in practice, bycirculating in the reaction vessel a gaseous mixture havingsubstantially constant characteristics, formed for the most part ofrecycled gaseous mixture.

In this way it is possible to produce under very satisfactory industrialconditions, a large number of polymers of ethylene, in particle form,including polyethylenes and copolymers of ethylene with higheralpha-olefins, such as high density polyethylene (density comprisedbetween 0.970 and greater than or equal to 0.940), amongst which may bementioned the homopolymers of ethylene and the copolymers of ethyleneand higher alpha-olefins comprising 3 to 12 carbon atoms, having acontent by weight of units derived from ethylene of greater than orequal to 97%, or linear low density polyethylene (density comprisedbetween 0.900 and 0.940), consisting of copolymer of ethylene and atleast one higher alpha-olefin comprising 3 to 12 carbon atoms, having acontent by weight of units derived from ethylene comprised between 80and 97%. The chromium content in the polymers or in the copolymers isgenerally less than 20 ppm and usually less than 5 ppm.

The polyethylene and ethylene copolymer powders prepared by the gasfluidised bed polymerisation process of the present invention have amean particle diameter in the range 100 to 4000 microns, preferably 300to 1600 microns and most preferably 500 to 1200 microns. The powder bulkdensities are generally in the range 0.35 to 0.53 gcm⁻³, and preferably0.40 to 0.50 gcm⁻³.

The polymers of ethylene and copolymers of ethylene and higheralpha-olefin, obtained according to the present invention have theadvantage of possessing a broad distribution of molecular weights. Thismay be characterised by the ratio between the mean molecular weight bymass, Mw, and the mean molecular weight by number, Mn, of the polymersor copolymers measured by gel permeation chromatography (GPC), thisratio usually being greater than 6. This distribution of the molecularweights may also be characterised by the flow parameter, n, which isgreater than 1.8 and generally greater than 2.0, this flow parameterbeing calculated according to the following equation:

    n=log(MI.sub.21.6 /MI.sub.8.5)/log 21.6/8.5)

in which MI₂₁.6 and MI₈.5 are melt indices of the polymers or copolymersmeasured at 190° C. under a load of 21.6 kg (ASTM.D 1238-57 T, conditionF) and 8.5 kg respectively.

Measurement of the molecular weight distribution

The molecular weight distribution of a polymer or copolymer iscalculated by the ratio of the mean molecular weight by mass, Mw, to themean molecular weight by number, Mn, of the polymer or copolymer, from adistribution curve of the molecular weights obtained by means of a gelpermeation chromatograph of the "DU PONT" make, type "860" (HighTemperature Size Exclusion Chromatograph), provided with a pump of the"DU PONT" make, type "870", the operating conditions being as follows:

solvent: trichloro-1,2,4-benzene

solvent throughput: 0.8 ml/minute

three columns of "DU PONT" manufacture with "Zorbax" packing, with aparticle size of 6 microns and a porosity of 60 A, 1000 A, and 4000 Arespectively are used

temperature: 150° C.

sample concentration: 0.15% by weight

injection of volume: 300 ml

infra-red detection, at a wave length of 3425 microns, by means of acell 1 mm thick,

standardisation by means of a high density polyethylene sold by BPChimie SA under the trade name "Natene 6055" (R): Mw=70 000 andMw/Mn=3.8.

Method for the determination of the mean diameter by mass (Dm) of theparticles of support, catalyst, prepolymer, and polymer or copolymer

The mean diameter by mass (Dm) of the particles of support, catalyst,prepolymer and polymer or copolymer is measured from microscopeobservations by means of the OPTOMAX image analyser (Micro-MeasurementsLimited - Great Britain). The measuring principle consists in obtainingfrom the experimental study by optical microscopy of a population ofparticles, a table of absolute frequency giving the number (n_(i)) ofparticles belonging to each class (i) of diameters, each class (i) beingcharacterised by an intermediate diameter (d_(i)), comprised between thelimits of the said class. According to approved French standard NF X11-630 of June 1981, Dm is provided by the following formula: meandiameter by mass: ##EQU1##

Measurement by the OPTIMAX image analyser is performed by means of aninverted microscope which makes it possible to examine suspensions ofsupport particles, of catalyst, of prepolymer, or polymer or ofcopolymer at an enlargement comprised between 16 and 200 times. Atelevision camera picks up the images given by the inverted microscopeand transmits them to a computer which analyses the images line by lineand dot by dot on each line, with a view to determining the dimensionsor diameters of the particles and then classifying them.

The polymer or copolymer densities throughout this specification are thedensities as measured according to ASTM D 1505.

The following non-restrictive Examples illustrate the invention.

EXAMPLE 1 Preparation of the catalyst

For 5 hours a catalyst powder sold by Joseph Crosfield and Sons(Warrington, Great Britain) under the trade name "EP 20" (R)_(is)subjected to thermal treatment at 815° C. in a fluidised bed reactor bymeans of a current of dry air. This catalyst consists of chromium oxide,(CrO₃), associated with a silica support and containing 1% by weight ofchromium. After treatment and cooling to ambient temperature (20° C.),the catalyst (A) obtained occurs in the form of a powder consisting ofparticles with a mean diameter by mass of 125 microns. It is maintainedunder an atmosphere of nitrogen during storage.

Prepolymerisation in suspension

Into a 1000 liter stainless steel reactor, equipped with a stirrersystem rotating at 140 revolutions per minute, there are introducedunder an atmosphere of nitrogen 500 liters of n-hexane heated to 75° C.,then 264 millimoles of diethylaluminiumethoxide and 5.5 kg of catalyst(A) prepared previously. Ethylene is then introduced at a throughput of15 kg/h, for 4 hours. At the end of this period the prepolymersuspension thus obtained is maintained at a temperature of 75° C. for 30minutes in order to consume to the maximum extent the ethylene which hasnot reacted. The reactor is then degassed, then cooled to 60° C.

300 liters of n-hexane previously heated to 60° C. are added to theprepolymer suspension which is maintained under these conditions, withstirring, for 15 minutes before extracting from this suspensionapproximately 300 liters of liquid phase. This operation is repeatedtwice, then the prepolymer suspension is cooled to ambient temperature(20° C.) and 2120 millimoles of triethylaluminium are added to it. Afterdrying under nitrogen, approximately 60 kg of prepolymer (B) areobtained in the form of a powder consisting of particles with a meandiameter by mass of 190 microns and containing 1.6×10⁻² milligram atomsof chromium per gram and less than 2.0% by weight of polymer soluble inn-hexane at 60° C.

Fluidised bed polymerisation

One operates by means of a stainless steel fluidised bed reactor with adiameter of 45 cm, fluidisation being provided by a rising gas mixturepropelled at a velocity of 82 cm/s, at a temperature of 96° C.; this gasmixture consists of nitrogen and ethylene, the partial pressure (pp) ofthese ingredients being as follows:

pp nitrogen=0.95 MPa

pp ethylene=1.05 MPa.

There are introduced into this reactor 70 kg of an anhydrouspolyethylene powder which is carefully degassed, as charge powder. Thereare then introduced into this reactor in a sequential manner, at afrequency of once every 5 minutes, 4.7 grams of prepolymer (B) asprepared previously.

At the end of about 3 hours, when the polyethylene production has becomeregular, there are introduced continuously into the reactor 10 ml/hr ofa molar solution of tri-n-octylaluminium in n-hexane. By means ofsequenced withdrawal, 28 kg/hr of polyethylene powder are collected inorder to maintain the fluidised bed at a constant height in the reactor.After 8 hours of operation under these conditions, the charge powderintroduced initially intothe reactor is practically completelyeliminated, and a polyethylene powder is obtained having the followingcharacteristics:

density: 0.950 (at 20° C.);

chromium content: less than 2 ppm;

high load melt index (MI₂₁.6), measured at 190° C. under a load of 21.6kg: 2.0 g/10 minutes;

bulk density: 0.43 g/cm³ ;

mean diameter by mass of the particles: 900 microns;

flow parameter, n: 2.5;

unsaturation level of vinyl, vinylidene and vinylene types respectivelyequal to 0.111, 0.013 and 0.005 per 100 atoms of carbon.

EXAMPLE 2 Preparation of the catalyst

A catalyst comprising 1% by weight of chromium as a chromium oxidehaving the formula CrO₃ and 2% by weight of titanium as a titanium oxidehaving the formula TiO₂, associated with a grannular support of silicahaving a high pore volume above 1.8 ml/g is obtained after a heattreatment at 850° C. during 5 hours in a fluidised bed reactor, by meansof a current of dry air. The catalyst obtained (C) is cooled to ambienttemperature (20° C.) and occurs in the form of a powder consisting ofparticles having a mean diameter by mass of 150 microns. It ismaintained under an atmosphere of nitrogen during storage.

Prepolymerisation in suspension

One operates exactly as in Example 1, except for the fact that insteadof using the catalyst (A), one uses the catalyst (C). About 60 kg ofprepolymer (D) are obtained having a mean diameter by mass of 300microns and containing 1.7×10⁻² milligram atoms of chromium per grammeand less than 2.0% by weight of polymer soluble in n-hexane at 60° C.

Fluidised bed copolymerisation

One operates by means of a stainless steel fluidised bed reactor with adiameter of 45 cm, fluidisation being provided by a rising gas mixturepropelled at a velocity of 82 cm/sec, at a temperature of 98° C.; thegas mixture consists of hydrogen, ethylene and nitrogen, the partialpressures (pp) of these ingredients being as follows:

pp hydrogen: 0.36 MPa

pp ethylene: 1.05 MPa

pp nitrogen: 0.593 MPa.

Into this reactor there are introduced 70 kg of an anhydrouspolyethylene powder which is carefully degassed as charge powder. Thereare then introduced into this reactor in a sequence at the frequency ofonce every 5 minutes, 4 g of the prepolymer (D).

At the end of about 3 hours, when the polyethylene production has becomeregular, there are introduced continuously into the reactor 10 ml/hr ofa molar solution of tri-n-octylaluminium in n-hexane. By means ofsequenced withdrawal, 28 kg/hr of polyethylene powder are collected inorder to maintain the fluidised bed at a constant height in the reactor.After 8 hours of operation under these conditions, the charge powderintroduced initially into the reactor is practically completelyeliminated, and a polyethylene powder is obtained having the followingcharacteristics:

density: 0.953 (at 20° C.);

chromium content: less than 2 ppm;

melt index (MI₈.5), measured at 190° C. under a load of 8.5 kg: 1.5 g/10minutes;

bulk density: 0.43 g/cm³ ;

mean diameter by mass of the particles: 900 microns;

molecular weight distribution, Mw/Mn: 7.7;

flow parameter, n: 2.15;

unsaturation level of vinyl, vinylidene and vinylene types respectivelyequal to 0.113, 0.014 and 0.006 per 100 atoms of carbon.

EXAMPLE 3 Fluidised bed polymerisation

One operates exactly as in Example 2, except for the fact thefluidisation is provided by a gas mixture consisting of hydrogen,ethylene and nitrogen, the partial pressure (pp) of these threeingredients being as follows:

pp hydrogen: 0.704 MPa

pp ethylene: 1.05 MPa

pp nitrogen: 0.246 MPa

A polyethylene powder is obtained having the following characteristics:

density: 0.953 (at 20° C.);

chromium content: less than 2 ppm;

melt index MI₈.5 : 3 g/10 minutes;

bulk density: 0.40 g/cm³ ;

mean diameter by mass of the particles: 1500 microns;

Mw/Mn 7.5;

flow parameter, n: 2.10;

unsaturation level of vinyl, vinylidene and vinylene types respectivelyequal to 0.115, 0.012 and 0.008 per 100 atoms of carbon.

EXAMPLE 4 Fluidised bed copolymerisation

One operates by means of a stainless steel fluidised bed reactor with adiameter of 45 cm, fluidisation being provided by arising gas mixturepropelled at a speed of 82 cm/sec, at a temperature of 90° C.; the gasmixture consists of hydrogen, ethylene, 1-butene and nitrogen, thepartial pressures (pp) of these four ingredients being as follows:

pp hydrogen: 0.204 MPa

pp ethylene: 1.05 MPa

pp 1-butene: 0.021 MPa

pp nitrogen: 0.725 MPa

Into this reactor there are introduced 70 kg of an anhydrouspolyethylene powder which is carefully degassed, as charge powder. Thereare then introduced into this reactor in a sequence at the frequency ofonce every 5 minutes, 3.2 g of the prepolymer (D) prepared in Example 2.

At the end of about 3 hours the production of copolymer of ethylene and1-butene has become regular, and there are introduced continuously intothe reactor 10 ml/hr of a molar solution of tri-n-octylaluminium inn-hexane. By a sequenced withdrawal one collects about 30 kg/hr ofcopolymer powder in order to maintain the fluidised bed at a constantheight in the reactor. After 8 hours of operation under theseconditions, the charge powder initially introduced into the reactor ispractically completely eliminated and a powder is obtained of copolymerof ethylene and 1-butene having the following characteristics:

density: 0.938 (at 20° C.);

content by weight of units derived from 1-butene: 1.3%

chromium content: less than 2 ppm:

melt index, MI₈.5 : 6.1 g/10 minutes;

bulk density: 0.38 g/cm³ ;

mean diameter by mass of the particles: 1600 microns;

Mw/Mn: 7

flow parameter, n: 2.10;

unsaturation level of vinyl, vinylidene and vinylene types respectivelyequal to 0.118, 0.015 and 0.005 per 100 carbon atoms.

EXAMPLE 5 Preparation of the catalyst

For 5 hours a catalyst powder sold by Joseph Crosfield and Sons(Warrington, Great Britain) under the trade name "SD 575" (R) issubjected to thermal treatment at 800° C. in a fluidised bed reactor bymeans of a current of dry air. This catalyst consisting of chromiumoxide, of the formula CrO₃, associated with a silica support, contains1% by weight of chromium. After treatment and cooling to ambienttemperature (20° C.), the catalyst (E) obtained occurs in the form of apowder consisting of particles having a mean diameter by mass of 87microns. It is maintained under an atmosphere of nitrogen during itsstorage.

Prepolymerisation in suspension

Into a 1000 liter reactor in stainless steel, equipped with a stirrersystem rotating at 140 revolutions per minute, there are introducedunder an atmosphere of nitrogen 500 liters of n-hexane heated to 75° C.,then 290 millimoles of diethylaluminiumethoxide and 5.5 kg of catalyst(E) prepared previously. Ethylene is then introduced at a flow rate of15 kg/h, for 4 hours and 25 minutes. At the end of this period theprepolymer suspension thus obtained is maintained at a temperature of75° C. for 30 minutes in order to consume to the maximum extent theethylene which has not reacted. The reactor is then degassed, thencooled to 60° C.

300 liters of n-hexane previously heated to 60° C. are added to theprepolymer suspension which is maintained under these conditions, withstirring, for 15 minutes before extracting from this suspensionapproximately 300 liters of liquid phase. This operation is repeatedtwice, then the prepolymer suspension is cooled to ambient temperature(20° C.) and 288 millimoles of triethylaluminium are added to it. Afterdrying under nitrogen, approximately 60 kg of prepolymer (F) areobtained in the form of a powder consisting of particles with a meandiameter by mass of 255 microns and containing 1.75×10⁻² milligram atomsof chromium per gram and less than 2.0% by weight of polymer soluble inn-hexane at 60° C.

Fluidised bed polymerisation

One operates by means of a stainless steel fluidised bed reactor with adiameter of 45 cm, fluidisation being provided by a rising gas mixturepropelled at a velocity of 60 cm/s, at a temperature of 106° C.; thisgas mixture consists of hydrogen, ethylene and nitrogen, the partialpressures (pp) of these three ingredients being as follows:

pp hydrogen=0.96 MPa

pp ethylene=0.80 MPa

pp nitrogen=0.24 MPa.

There are introduced into this reactor 70 kg of an anhydrouspolyethylene powder which is carefully degassed as charge powder. Thereare then introduced into this reactor in a sequential manner, at afrequency of once every 7 minutes, 8.5 grams of prepolymer (F) asprepared previously.

At the end of about 3 hours, when the polyethylene production has becomeregular, there are introduced continuously into the reactor 80 ml/hr ofa 0.05 molar solution of tri-n-octylaluminium in n-hexane. By means ofsequenced withdrawal, one collects about 30 kg/hr of polyethylenepowder, which maintains the fluidised bed at a constant height in thereactor. After 8 hours of operation under these conditions, the chargepowder introduced initially into the reactor is practically completelyeliminated, and a polyethylene powder is obtained having the followingcharacteristics:

density: 0.954 (at 20° C.);

chromium content: 2.5 ppm;

melt index (MI₂₁.6), measured at 190° C. under a charge of 21.6 kg: 21g/10 minutes;

bulk density: 0.49 g/cm³ ;

mean diameter by mass of the particles: 1000 microns;

width of distribution of molecular weights, Mw/Mn: 7.3;

flow parameter, n: 2.0.

EXAMPLE 6 Fluidised bed copolymerisation

One operates by means of a stainless steel fluidised bed reactor with adiameter of 45 cm, fluidisation being provided by a rising gas mixturepropelled at a speed of 70 cm/sec, at a temperature of 92° C.; the gasmixture consists of hydrogen, ethylene, 1-butene and nitrogen, thepartial pressures (pp) of these four ingredients being as follows:

pp hydrogen: 0.52 MPa

pp ethylene: 0.80 MPa

pp 1-butene: 0.036 MPa

pp nitrogen: 0.64 MPa.

Into this reactor there are introduced 70 Kg of an anhydrouspolyethylene powder which is carefully degassed, as charge powder. Thereare then introduced into this reactor in a sequence at the frequency ofonce every 15 minutes, 8.5 g of the prepolymer (F) prepared in Example5.

At the end of about 3 hours the production of copolymer of ethylene and1-butene has become regular, and there are introduced continuously intothe reactor 50 ml/hr of a 0.05 molar solution of tri-n-octylaluminium inn-hexane. By a sequenced withdrawal one collects about 30 kg/hr ofcopolymer powder which permits to maintain the fluidised bed at aconstant height in the reactor. After 8 hours of operation under theseconditions, the charge powder initially introduced into the reactor ispractically completely eliminated and a powder is obtained of copolymerof ethylene and 1-butene having the following characteristics:

density: 0.918 (at 20° C.);

content by weight of units derived from 1-butene: 7.2%

chromium content: less than 2 ppm:

melt index, MI₂.16 : 0.52 g/10 minutes;

bulk density: 0.35 g/cm³ ;

mean diameter by mass of the particles: 1100 microns;

Mw/Mn: 6.1

flow parameter, n: 1.90;

unsaturation level of vinyl, vinylidene and vinylene types respectivelyequal to 0.06, 0.02 and 0.005 per 100 carbon atoms.

We claim:
 1. A prepolymer of ethylene, or a prepolymer of ethylene andat least one higher alpha-olefin, prepared in a prepolymerisation stepby contacting ethylene monomer or a mixture of monomers of ethylene andat least one higher alpha-olefin with a catalyst comprising a chromiumoxide associated with a granular refractory oxide support, characterizedin that the prepolymer containing said catalyst in an active form andconsisting of solid particles having a chromium content from 4×10⁻⁵ to 3milligram atoms of chromium per gram and having a mean diameter by masscomprised between 40 and 1000 microns is free from any monomer afterhaving been separated from the monomer(s) of the prepolymerisation step,so that said prepolymer has an initial catalytic activity substantiallyreduced when it is used in a gas phase (co-)polymerisation of ethylene.2. A prepolymer according to claim 1 wherein the higher alpha-olefincomprises 3 to 12 carbon atoms.
 3. A prepolymer according to claim 1,wherein the higher alpha-olefin is propylene, 1-butene, 1-hexene,4-methyl-1-pentene or 1-octene.
 4. A prepolymer according to claim 1wherein the prepolymer also contains at least one organometalliccompound (a) of a metal of groups I to III of the Periodic Table ofElements, in a quantity such that the atomic ratio of the quantity ofmetal of said organometallic compound (a) to the quantity of chromium isfrom 0.01 to
 30. 5. A prepolymer according to claim 1 wherein theprepolymer contains an organometallic compound (b) of a metal of groupsI to III of the Periodic Table of Elements, in a quantity such as theatomic ratio of the quantity of metal of said organometallic compound(b) to the quantity of chromium is at most equal to
 100. 6. A prepolymeraccording to claim 1 wherein the prepolymer contains an organometalliccompound (a) and an organometallic compound (b) of metals of groups I toIII of the Periodic Table of Elements, the organometallic compound (b)being identical to or different from the organometallic compound (a), inquantities such as the atomic ratio of the quantities of metals of saidorganometallic compounds (a) and (b) to the quantity of chromium is atmost equal to
 130. 7. A prepolymer according to claim 1 in the form of adry powder.
 8. A prepolymer according to claim 1 wherein the chromiumcontent is from 10⁻³ to 10⁻¹ milligram atoms of chromium per gram.
 9. Aprepolymer according to claim 1 wherein the prepolymer has a content ofpolymer or copolymer soluble in n-hexane at 60° C. of less than or equalto 2.0% by weight.